A laboratory animal model for the human lymphatic-dwelling filaria, Wuchereria bancrofti has been lacking. To date, infections have been established in 6 of 11 patas monkeys inoculated with infective stage larvae of the parasite. The first 10 patas were inoculated with larvae from mosquitoes that fed on infected human blood. The most recent was an animal inoculated with larvae collected from Culex quinquefasciatus (Haiti strain) mosquitoes that were infected by blood feeding directly on another patas with circulating microfilariae. The aim was to passage the parasite in monkeys in the hope that it would become better adapted to the monkey host. The monkey developed an uncommonly high microfilaremia (95 mf per 20 fl), but despite this, the infectivity rate of mosquitoes that blood fed on it was low. Whether this was due to the development of resistance in the Haiti mosquito strain or diminished infectivity of the microfilariae due to passage was investigated. The Covington strain of Cx. quinquefasciatus was established as a control. A portion of both mosquito strains was blood fed directly on the monkey, and the remainder were fed on infected blood from a Haitian patient (cryopreserved blood). Only a few larvae developed in the Haiti strain of the mosquito after feeding on both the monkey and the human blood. For the Covington mosquito strain, only a few larvae developed after feeding on the monkey blood, but numerous larvae were recovered from mosquitoes that fed on the human blood. These results indicate that the Haiti strain of Cx. quinquefasciatus, which now is 178 passages from the field, has indeed become refractory to infection, but more importantly it shows that passage of the parasite in a monkey has considerably diminished the ability of microfilariae to infect mosquitoes.